Among the aromatic polyimides, poly(4,4'-oxydiphenylenepyromellitimide) is widely used in films, coating materials, moldings and the like, as a polymer having excellent heat resistance and dynamic characteristics.
Journal of Polymer Science, Macromolecular Reviews, Vol. 11 (1976) discloses, in Table 2 on page 164, solvents capable of dissolving poly(4,4'-oxydiphenylenepyromellitamic acid) which is a typical precursor of poly(4,4'-oxydiphenylenepyromellitimide). Listed in this table as illustrative solvents are N,N-dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP), hexamethylphosphoramide (HMPA), N-methylcaprolactam, dimethyl sulfoxide (DMSO), N-acetyl-2-pyrrolidone, N,N-dimethylacetamide (DMAc) and the like. These solvents can dissolve the polyimide precursor when used alone and are called aprotic polar solvents. In addition, this reference discloses that these solvents can be used not only for dissolving a poly(amic acid) but also as a polymerization solvent when a poly(amic acid) is produced by the polymerization of a diamine with a tetracarboxylic acid dianhydride.
This same reference also discloses, on pages 199 to 205, that polyimide films can be obtained by removing solvents from solutions of poly(amic acid) dissolved in these aprotic polar solvents and carrying out imidization, and that polyimide-coated materials can be obtained by coating base materials with these solutions, followed by solvent removal and imidization.
Polyimide precursor solutions prepared using DMAc, NMP, DMSO, DMF and the like aprotic polar solvents are also disclosed in U.S. Pat. No. 4,238,528, JP-B-3-4588 (The term "JP-B" as used herein means an "examined Japanese patent publication") and IBM Technical Disclosure Bulletin, Vol.20, no.6, p.2041 (November, 1977). However, since these solvents have a large dipole moment, associate strongly with polyimide precursors used as solutes (Journal of Polymer Science, A-1, Vol. 4, pp. 2607-2616, 1966, Journal of Polymer Science, A, Vol. 25, pp. 2005-2020, 1987, Journal of Polymer Science, A, Vol. 25, pp. 2479-2491, 1987, Kogyo Kagaku Zasshi, Vol. 71, no. 9, pp. 1559-1564, 1968, and Abstract of ANTEC '91, pp. 1742-1745) and have strong solvation with the solutes used, they cause the following problems when polyimide films or coated materials are produced. That is, in a solution prepared by dissolving a polyimide precursor in the aforementioned currently used aprotic polar solvent, the polyimide precursor and the solvent are strongly solvated, thus leading to difficulty in removing the solvent at the time of molding or coating and therefore causing a problem in terms of insufficient dynamic and electric characteristics of the resulting moldings or coated materials due to the large amount of remaining solvent. In addition, the remaining solvent in the moldings causes another problem in that it generates toxic gases such as carbon monoxide and the like upon its decomposition caused by high temperature at the time of the use of the product.
Also, since they are hygroscopic in nature, these solvents pose still another problem when they are used as a polymerization solvent. That is, when water is present in the solvent, it causes hydrolysis of acid anhydrides and changes in the polyimide precursor solution formed with the passage of time, thus requiring reaction in a strictly water-free system which requires a complex reaction apparatus. In addition, since these solvents have a large surface tension due to their large dipole moment and a high viscosity as described in JP-B-3-4588, a sufficiently uniform coating cannot be obtained in a film or a coated material, and such a coating has insufficient adhesion to base materials.
Contrary to this, the inventors of the present invention found an important solvent system which can resolve many problems inherent in these aprotic polar solvents, and filed a patent application on a polyimide precursor solution that does not use aprotic polar solvents (JP-A-6-1915).
Specifically, the present inventors found that a solution of a polyimide precursor having a high polymerization degree can be produced easily at low cost even if water is present when a mixture of solvents, which individually do not solvate strongly with the polyimide precursor when used alone, is used as a polymerization solvent, that the polyimide precursor in this solution does not solvate strongly with the solvent and that polyimide moldings such as strings, films and the like, as well as coated materials, having excellent characteristics can be obtained from this solution. However, when a solution prepared using these solvents-was spread on a base material to remove the solvents and then subjected to imidization, the resulting film was not a porous film but a film having no pores.
Polyimide porous films are used as gas or liquid separation films. JP-A-57-170934 and JP-A-57-170935 (The term "JP-A" as used herein means an "unexamined published Japanese patent application") disclose polyimide porous films obtained by the use of polyimide solutions which comprise polyimides prepared from benzophenonetetracarboxylic acid or biphenyltetracarboxylic acid and an aromatic diamine and good and poor solvents of these polyimides, and processes for the production of these films. However, since polyimide porous films obtained by these processes are derived from solvent-soluble polyimides, they are inferior to poly(4,4'-oxydiphenylenepyromellitimide) films in terms of heat and chemical resistances and therefore can be used only for a limited purpose.
JP-B-61-53086 also discloses a process for the production of a polyimide porous film in which a film formed from a solution of a poly(amic acid) dissolved in the aforementioned aprotic polar solvent is allowed to contact a coagulation solution comprising water or a lower alcohol, and the thus formed coagulation film is dried and subjected to heat imidization. According to this process, a polyimide porous film containing poly(4,4'-oxydiphenylenepyromellitimide) having excellent heat and chemical resistances can be obtained, but it shows insufficient gas permeability (in the case of hydrogen for example, its gas permeability is merely from 0.0016 to 0,011 cm.sup.3 /cm.sup.2 .multidot.sec.multidot.cmHg).
In addition, JP-A-57-170936 discloses polyimide porous films, as well as a process for the production thereof, which porous films are obtained by preparing a polyimide precursor film using a solution of a polyimide precursor dissolved in a mixture of good and poor solvents for the precursor, and heating the thus prepared precursor film to effect removal of the solvents and imidization of the film. This unexamined published patent application states that these polyimide porous films are useful as gas separation films, because these films are composed of poly(4,4'-oxydiphenylenepyromellitimide) and the like and therefore have excellent heat and chemical resistance.
However, since the process of JP-A-57-170936 uses the aforementioned aprotic polar solvents which are good solvents of the polyimide precursor, it has various problems inherent with these aprotic polar solvents. In addition, since aprotic polar solvents which remain in the film soften the polyimide precursor film when the film is subjected to heating to effect a ring closure reaction, movement of the polymer molecules becomes frequent, which causes rearrangement of the molecules, as has been noted in Polymer Engineering Science, Vol.29, pp.347-351 (1989) and Advances in Polyimide Science and Technology, Tecnomic Publishing Co., Inc., pp.360-373 (1991).
Because of such softening of the film and rearrangement of the polymer molecules, the fine porous structure once formed at the time of polyimide precursor film preparation ceases to exist, resulting in difficulty in obtaining sufficient gas permeability, as has been pointed out in Membrane, Vol.15, pp.139-146 (1990).
For the purpose of overcoming this problem, Membrane, Vol.15, pp.139-146 (1990) and Membrane, Vol.17, pp.42-47 (1992) disclose a process in which a polyimide porous film comprising poly(4,4'-oxydiphenylenepyromellitimide) is obtained for use as a gas separation film, by carrying out heat treatment in high boiling point solvents to effect imidization while maintaining the fine porous structure formed at the time of the preparation of the polyimide precursor film. However, the gas permeability cannot be improved to a satisfactory level even by the use of this process.
In addition, JP-B-58-25690 discloses a process in which a polyimide porous film is produced by forming a polyimide precursor film on a glass plate using a polyimide precursor solution and soaking the resulting plate in a liquid which chemically converts the precursor into the corresponding polyimide. However, the gas permeability cannot be improved to a satisfactory level by the use of this process.
Since they exhibit excellent electrical characteristics, polyimides are also useful as electronic materials such as insulating layers for semiconductor elements and insulating films for flexible circuit boards. However, with the recent large scale integration of semiconductor elements and circuits and high-speed signal systems, great concern has been directed toward the development of materials having more excellent electrical characteristics, including dielectric characteristics. An improvement of dielectric characteristics may be achieved, for example, by a process in which a polyimide film is made into a porous structure by introducing fine pores into the film. Such a process can decrease the dielectric constant of the film while maintaining full characteristics of the polyimide. For example, Advances in Polyimide Science and Technology, Tecnomic Publishing Co., Inc., pp.184-197 (1991) discloses a process in which a porous film is produced by first molding a film from a solution of a copolymer consisting of a polyimide having a specified structure and polyethylene oxide and then subjecting the film to sintering to effect decomposition and removal of polyethylene oxide. Though this reference states that the porous film obtained by this process is useful as a semiconductor insulating film, this porous film cannot be put into practical use because of its inferior solvent resistance due to the use of a solvent soluble polyimide.
A polyimide film whose dielectric constant is reduced by changing its chemical structure is known for use as an insulating film for flexible circuit boards. Though dielectric characteristics can be improved by changing the chemical structure of the film, this change results in a decrease in other characteristics. A reduction of the dielectric constant by providing a polyimide film for flexible circuit board width a porous structure seems to be very effective, but no information is available to date concerning a polyimide porous film which comprises poly(4,4'-oxydiphenylenepyromellitimide) and which has excellent dielectric characteristics and sufficient dynamic characteristics.
As has been described in the foregoing, there are no porous films composed of poly(4,4'-oxydiphenylenepyromellitimide) which have markedly excellent heat and chemical resistance and which can be produced without using aprotic polar solvents such as DMF, NMP, DMAc, DMSO and the like, which solvents are disadvantageous from various points of view.